Nearly thirty million people in the United States suffer from impaired hearing or balance. The majority of these individuals experience "sensorineural" deficits, which largely ensue from the derangement or death of the ear's sensory receptors, the hair cells. The proposed experiments are meant to provide a better understanding of the normal transduction process by which hair cells capture mechanical stimuli - sounds in the cochlea, and accelerations in the vestibular apparatus - and transform them into electrical responses that can be interpreted by the brain. The long-term goal of the studies is to recognize how hearing and balance are impaired by genetic abnormalities, infections, ototoxic drugs, traumatic sounds, and ageing, and thus to develop rational strategies for the prevention or amelioration of hearing and balance problems. The experiments proposed in this application deal with five principal issues. Each hair cell possesses an adaptation mechanism that constantly sets its sensitive hair bundle in an optimal position to respond to mechanical stimulation. It has been proposed that adaptation involves the activity of a molecular motor, perhaps a member of the myosin family. The first proposed studies are meant to determine precisely the forces produced by the putative motor during adaptation. A second, related set of experiments will explore the sensitivity of the motor to nucleotide analogues that are expected to interfere with the activity of myosin-like molecules. A third topic to be explored is the contribution of hair bundles to the mechanical properties of the receptor organs in which they reside. The studies planned in this context should indicate whether hair bundles could influence the basilar membrane's motion in the cochlea and the movement of otolithic membranes and cupulae in the vestibular labyrinth. Under in vitro conditions, a hair bundle can display rapid, twitching motions triggered by mechanical inputs. These transients are the fourth subject of investigation: how do they arise, what is their timecourse and dependence upon stimulation, and are they energetic enough to contribute to amplification of stimuli in the ear? In addition to responding to mechanical inputs, hair cells have two other important roles in information processing: they effect tuning by selecting inputs at frequencies of greatest behavioral relevance, and they synaptically transmit signals to afferent nerve fibers. The final topic of the proposed research deals with the ion channels key to both processes, voltage-activated Ca2+ channels on a hair cell's basolateral surface. An effort will be made to determine whether clusters of these channels occur at the sites of transmitter release. The results will clarify how Ca2+ entry mediates transmitter release, and aid in our understanding of frequency tuning by electrical resonance.